Solid solution formation of Sm 2؉ and MSO 4 (M ؍ Sr, Ba) was investigated for syntheses using (i) a LiCl high-temperature solution exposed to a reducing atmosphere and (ii) precipitation reactions at room temperature starting from aqueous solutions of SmI 2 or electrochemically gained Sm 2؉ . In contrast to (ii), present results show that a high-temperature approach (i) yielded only a very low amount of Sm 2؉ in M 1؊x Sm x SO 4 . Formation of a solid solution system (0 < x < 1) was con5rmed for Sr 1؊x Sm x SO 4 and Ba 1؊x Sm x SO 4 by X-ray powder di4raction analysis and optical lifetime measurements. The unit cell parameters of Ba 1؊x Sm x SO 4 showed a slight deviation from Vegard:s law. Positive and negative deviations are in agreement with results on solid solutions of Ba 1؊x Sr x SO 4 . Compounds obtained by syntheses at room temperature were exposed to annealing at 450 to 8503C using a reducing or oxidizing atmosphere. In this temperature range, M 1؊x Sm x SO 4 (M ؍ Sr, Ba) decomposed into Sm 2 O 2 (SO 4 ) and the corresponding MSO 4 . Solid solutions of M 1؊x Sm x SO 4 (M ؍ Ba, Sr) represent a new system for investigating Sm 2؉ in an oxide environment. There are only a few other oxide host lattices stabilizing divalent samarium. 2000 Academic Press